A program is underway to study biological alkylation reactions occurring in the fungal system Penicillium citrinum. This system produces the antibiotic, citrinin, which undergoes three C-1 transalkylation reactions during its biosynthesis. Methionine feeding studies suggest that each alkylation follows a different pathway, or at least requires enzymes and additional information on the mechanisms of alkylation operating in this system has been forthcoming from selected inhibition studies and feeding experiments using potential metabolic precursors. A concurrent investigation of the degree of N-methylation of soluble RNA and how such methylation is influenced by antimetabolites is also underway. At the present time it is believed that abnormal alkylation of soluble RNA bases may play a role in the growth of neoplastic cells.